In a genomic approach, an androgen-regulated gene, which is predominantly expressed in the submandibular gland (SMG) and prostate of adult rats, has been identified (Rosinski-Chupin et al., 1988 and European patent 0 394 424). The gene encodes a precursor protein, submandibular rat1 protein (SMR) giving rise to three structurally related peptides which are selectively matured from the precursor in vivo by cleavage at multibasic sites by a paired basic amino acid-converting enzyme (Rougeot et al., 1994).
In an approach of post-genomic and physiomic, it was established the molecular and functional bases providing evidence for the existence in mammals of a hormonal messenger of the intercellular communication, i.e., the final mature peptide generated from SMR1 pre-prohormone: SMR1-Pentapeptide, named today Sialorphin (of sequence QHNPR (SEQ ID NO: 8)). Hence, sialorphin is an exocrine and endocrine peptide-signal, whose expression is under activational androgenic regulation and secretion is evoked under adrenergic-mediated response to environmental stress, in male rat (Rougeot et al., 1997).
The fact that, in sexually mature male rat, the androgen-regulated sialorphin is acutely secreted in response to environmental acute stress, led to postulate that this signaling mediator might play a role in some physiological and behavioral integration linked to the reproduction. Thus the same authors investigated the effects induced by sialorphin on the male sexual behavior pattern, which included frequency and latency of mounts, intromissions and ejaculations, as well as socio-sexual interactions. The data obtained showed that sialorphin has the ability to modulate, at doses related to physiological circulating levels, the male rat mating pattern, i.e., exerting, in a dose-dependent manner, a dual facilitative/inhibitory effect on the sexual performance, while stimulating at all doses the apparent sexual arousal or motivation. Thus it is proposed that the endogenous androgen-regulated sialorphin helps modulate the adaptative balance between excitatory and inhibitory mechanisms serving appropriate male rat sexual response, depending on the context.
International patent application WO 01/00221 describes the use of maturation products of SMR1 for the treatment of impaired interpersonal and behavioural disorders, including sexual defects.
Furthermore, these authors discovered that SMR1 maturation products recognize specific target sites in organs that are deeply involved in the mineral ion concentration. International patent application WO 98/37100 describes the therapeutic use of maturation products of SMR1 for preventing or treating diseases associated with a mineral ion imbalance in a human or an animal body.
In response to stressful contexts, sialorphin is acutely released, rapidly distributed and lasting taken up by its systemic membrane-associated targets (Rougeot et al., 1997). The authors have demonstrated that the major cell surface molecule to which sialorphin binds in vivo is the membrane-anchored metalloecto-endopeptidase, NEP (Neutral Endopeptidase; Neprilysin EC 3.4.24.11), or enkephalinase (Rougeot et al., 2003). Moreover, sialorphin was shown to be a physiological antagonist of the NEP activity ex vivo; and the direct interaction of NEP and sialorphin assessed in an in vitro assay using soluble purified renal NEP and artificial fluorogenic DGNPA (Dansyl-Gly-(pNO2)Phe-βAla) as substrate provided direct evidence that sialorphin inhibited NEP activity (IC 50 of the sialorphin: 0.6 μM). Sialorphin, is the first physiological inhibitor of the NEP-enkephalinase activity identified to date in rodent (Rougeot et al., 2003 and European patent application EP 1 216 707).
NEP is located at the surface of cells in nervous and systemic tissues, where it plays an important function as an ectoenzyme catalyzing the post-secretory processing or metabolism of a number of neuropeptides and regulatory peptides. The main physiologically relevant substrates for NEP are the enkephalins, substance P and atrial natriuretic peptide (ANP). These mammalian signal peptides are involved in the control of central and peripheral pain perception, inflammatory phenomena, arterial tone and mineral homeostasis. Their physiological importance and the critical role of NEP ectoenzyme in modulating their functional potency make it important to investigate and know their possible protection by endogenous inhibitors, from a physiological as well as a physiopathological and therapeutic point of view.
By using different models of molecular and behavioral pharmacology, the authors have shown that the physiological mediator, sialorphin, prevents spinal and renal NEP from breaking down its two physiologically relevant substrates, Substance P and Met-enkephalin in vitro. Sialorphin inhibited the breakdown of substance P with an IC50 of 0.4-1 μM and behaved as a competitive inhibitor of the membrane-bound NEP that originates from nervous tissues (spinal cord) or from systemically tissues (kidney, bone, tooth, placenta, prostate, GSM, intestine). In vivo, intravenous sialorphin elicited potent antinociceptive responses in two behavioral rat models of injury-induced acute and tonic pain, the pin-pain test (mechanical algesia) and formalin test (chemical algesia). The analgesia induced by sialorphin required the activation of μ- and δ-opioid receptors, consistent with the involvement of endogenous opioid receptors in enkephalinergic transmission. Indeed, these receptors are involved in the transmission of the endogenous opioidergic signals such as the enkephalins which are inactivated by NEP and the aminopeptidase APN, and also of the exogenous opiate, the morphine which interacts mainly with the μ-opioid receptor. It was concluded that the sialorphin protects endogenous enkephalins released following nociceptive stimuli by inhibiting ecto-enkephalinases, in vivo, and thus potentialises their analgesic effect. Otherwise, the endogenous opioid system, in particular δ-opioid-mediated pathway, has also been linked to the etiology of depressive behavior; for instance using a model of analysis of behavioral despair (forced swim test), the authors showed that sialorphin displays a significative antidepressant activity in male rat. Sialorphin is the first natural systemically active regulator of NEP activity identified to date in mammals. Furthermore, evidence was provided that it is a new physiological modulator of pain perception following injury, and may be the progenitor of a new class of therapeutic molecules, as putative novel antinociceptive and antidepressive agents (Rougeot et al., 2003; EP 1,343,519 and EP 1,343,520).
The powerful analgesic effect of sialorphin is associated to its capacity to entirely protect the enkephalins from inactivation by the enkephalin-degrading ectoenzymes. In vivo, the enkephalins are inactivated with an extraordinarily efficiency (within few seconds) by the both ectopeptidases, NEP and APN. In agreement, the first developed synthetic inhibitors, which are either only NEP specific (such as Thiorphan) or APN specific (such as Bestatin) exhibit a non-significant or weak antinociceptive effect. Thus, rat sialorphin is a physiological dual inhibitor of NEP and APN metallo-ectopeptidases; furthermore, this endocrine signal messenger of the adaptative response to stress is a powerful inhibitor of painful perception in rat and its analgesic effect is more potent than that of synthetic dual NEP/APN inhibitors such as kelatorphan, which have been developed elsewhere by modeling methods. So, sialorphin is remarkably adapted in terms of specificity and bioavailability to the conformational and distributive characteristics of its targets and as a consequence is more effective from an integrative point of view. Considering these observations, from a functional as well as physiopathological and therapeutic point of view, the biological importance of the functions regulated by the rat sialorphin makes it crucial to investigate and identify the endogenous functional homologous of rat sialorphin in human.
Sialorphin is the only identified physiological systemically active regulator of the membrane-bound enkephalinase activity in mammals. This raises the question of the existence of such endogenous NEP-ectopeptidase inhibitor in human saliva and blood. No immunoreactive QHNPR peptide (SEQ ID NO: 8) (sialorphin) was detected in male human saliva using highly sensitive and specific radioimmunoassay (Rougeot et al., 1994). However, bibliographical data let suppose the presence of low molecular weight substances (≦3000 Da), inhibiting the NEP ectopeptidase activity in human, notably in the human saliva. Although this(ese) salivary component(s) was(were) not biochemically characterized, a gender-related difference was observed in the salivary production of this(ese) inhibitor(s) of human enkephalin-degrading ectoenzymes (Marini and Roda, 2000). Strikingly, the situation is very similar to that one identified by the inventors in male rat, wherein the submandibular gland and the saliva represented the compartments of major synthesis and secretion of sialorphin, respectively.
The gene encoding the SMR1 precursor of sialorphin belongs to a multigene family whose members have been identified in human. However, the stricto sensu homologous human gene of rat SMR1 gene (VCSA1 coding for SMR1) was not found in human (cDNA cloning and human genome analysis). Furthermore, the inhibitory potency of rat sialorphin against membrane-anchored human NEP, which is expressed by human prostate cell lines (LNCaP), exists but is about 10-fold lower than that obtained against rodent NEP (rat, rabbit). This apparent selectivity in the functional interaction between rat sialorphin and NEP ectoenzyme is at least surprising considering the fact that the rat and human NEP have relatively high amino-acid sequence analogy (about 85%). Otherwise, the characterization of the human genes of the multigenic family to which belongs the gene coding for the precursor of the rat sialorphin (SMR1), revealed that it exists in human, several genes of this family, among which three were characterized, i.e., the genes hPB, hPBI and BPLP which are clustered in the same chromosome region, q13-21 of Chromosome 4 (Isemura, 2000) (Isemura and Saitoh, 1997) (Dickinson and Thiesse, 1996).